There may be a bigger concern. Who told you that if a machinegun fired only blanks it was legal? AFAIK, if it is or ever was a machinegun, even totally deactivated, it must be registered or it is illegal to own.
Making a full auto blank gun from a kit would likewise be illegal, as would making a semi-auto (blank or not) that fires from an open bolt.
BTW, as a general rule, blank guns are best made with no hole at all in the front of the barrel and an opening in the top well back from the muzzle. This protects against someone placing the blank gun to his head and pulling the trigger, which will seriously disrupt his brains shortly thereafter.
Yes, I know about movie guns, but the companies that rent them out have all the licenses and papers needed, plus most guns (especially machineguns) in the movies today are dummies. Some fire propane gas, but others are pure dummies and the sound and flash are added to the movie by the "magic" boys in the back room.
Jim